1. Field of the Invention
This invention relates to a method for the conversion of methane. More particularly, the invention relates to the photocatalytic conversion of methane to methanol.
2. Background of the Related Art
The ability to directly convert methane to methanol in economically satisfactory yields is important in many industries including the oil and gas industry. Methane is an abundant material found world-wide, particularly in the form of natural gas, which is difficult and costly to transport. Conversion of natural gas to methanol in a liquid form allows for safer, more efficient transportation. In addition, methanol may be used as a valuable commercial feedstock, an important ingredient in the production of reformulated motor fuels, and/or an environmentally compatible fuel in itself.
It is desirable to upgrade available methane to methanol or higher oxygen atom containing hydrocarbons, such as alcohols, ethers, aldehydes, etc. Existing technologies for converting methane to methanol include destruction of methane to form a synthesis gas (H.sub.2 and CO), followed by indirect liquefaction steps.
Production of alcohols and/or olefins by oxidation of organic compounds is difficult because the oxidation reaction tends toward completion to carbon dioxide. Overoxidation has been a persistent problem and arresting the oxidation at a desired intermediate oxidation product is a goal of much research.
However, conventional catalytic approaches to produce methanol from methane typically have poor conversion efficiencies, slow reaction rates, and are not economically competitive because they are typically very energy intensive. One such process, the oxidative coupling process, involves the use of an oxidant to abstract hydrogen from methane and coupling two or more hydrocarbon radicals to form light olefin, oxygenates, and other hydrocarbons. The oxidants are oxygen, halogens and reducible metal oxides as oxygen carriers and catalysts. In the oxidative coupling processes, hydrogen abstraction at the oxygen centers of the catalyst is typically the rate determining step, and catalyst properties are important for end product selectivity. Therefore, the maximum rate of product conversion strongly depends on the rate of radical formation on the active oxygen centers. In order to increase the rates, chemists have used high temperatures, even in excess of 900.degree. C. However, this undesirably promotes deep oxidation of methane to fully oxidized species, such as CO.sub.2. In another strategy, a high temperature dehydrogenation coupling process has a very high radical generation rate, and correspondingly a high rate of light olefin formation. However, the process is plagued by solid carbon formation which lowers the efficiency of the olefin production, and excess hydrogen is necessary to suppress the solid carbon formation.
One conventional method for the catalytic conversion of methane to methanol involves a first reaction with water to produce synthesis gas, which is a mixture of carbon monoxide and hydrogen, followed by catalytic conversion of the synthesis gas to methanol. A direct, one-step oxidation of methane to methanol would be simpler, and economically and environmentally preferable.
Therefore, there is a need for a process that produces methanol in a gas or liquid form. It would be desirable if the process was cost effective, easy to operate, relatively fast, and capable of achieving total mineralization.